1. Field of the Invention
The present invention relates to a display device and a method for manufacturing the display device.
2. Description of the Related Art
In recent years, a liquid crystal display device and an electroluminescent display device, in each of which thin film transistors (hereinafter also referred to as TFTs) are integrated over a glass substrate, have been developed. In each of these display devices, a thin film transistor is formed over a glass substrate by using a technique for forming a thin film, and a liquid crystal element or a light-emitting element (an electroluminescent (hereinafter, also referred to as an EL) element) is formed as a display element over various circuits composed of the thin film transistors so that the device functions as a display device.
A thin film transistor is a switching element that is turned on when a certain amount of voltage (referred to as a threshold or a threshold voltage) is applied to a gate electrode and is turned off when a voltage less than the amount is applied. Therefore, it is very important to control a threshold voltage precisely in terms of accurate operation of a circuit.
However, a threshold voltage of a TFT is moved (shifted) toward the minus side or the plus side by an indefinite factor such as an effect of a movable ion due to contamination and an effect of difference in work function and an interface charge in the periphery of a gate of the TFT.
As a technique proposed as a means for solving such a phenomenon, a channel doping method is given. The channel doping method is a technique in which an impurity element imparting one conductivity (typically, P, As, B, or the like) is added to at least a channel formation region of a TFT and a threshold voltage is controlled by being shifted intentionally (for example, see Patent Document 1: Japanese Published Patent Application No. 2003-257992).